linux-kernel - Re: [RFC PATCH -next v2 3/4] arm64/ftrace: support dynamically allocated trampolines

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Date:   Mon, 9 May 2022 14:22:03 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Mark Rutland <mark.rutland@....com>,
        Wang ShaoBo <bobo.shaobowang@...wei.com>,
        cj.chengjian@...wei.com, huawei.libin@...wei.com,
        xiexiuqi@...wei.com, liwei391@...wei.com,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        catalin.marinas@....com, will@...nel.org, zengshun.wu@...look.com
Subject: Re: [RFC PATCH -next v2 3/4] arm64/ftrace: support dynamically
 allocated trampolines

On Thu, 5 May 2022 12:15:38 +0900
Masami Hiramatsu <mhiramat@...nel.org> wrote:

> OK. But my interest is that the ftrace on arm64 can provide a limited
> access to registers via pt_regs or not. I don't mind the contained values
> so much because in the most case, 'users' will (most likely) access to the
> ARGs via BPF or tracefs (and we can just warn users if they try to access
> the registers which is not saved.) But if the arm64 ftrace only provides
> a special data structure, arch-independent code must have 2 different access
> code. That is inefficient. That is my concern.
> IOW, I'm interested in interface abstraction.

Note, ftrace now has a ftrace_regs structure that is passed to the
callbacks for the function tracer.

It then has an arch dependent helper function ftrace_get_regs(fregs), that
returns a pt_regs from the fregs only if the fregs has a full pt_regs to
return. If not, it returns NULL.

This was suggested by both Peter Zijlstra and Thomas Gleixner when I
introduced FTRACE_WITH_ARGS, where all functions can now get the arguments
from fregs, but not the full pt_regs. If a ftrace_ops has the REGS flag set
(using ftrace_regs_caller), the ftrace_get_regs(fregs) will return the
pt_regs, or it will return NULL if ftrace_regs_caller was not used.

This way the same parameter can provide full pt_regs or a subset, and have
an generic interface to tell the difference.

-- Steve